Field of the Invention
Disclosed herein are stents for implantation within the body and methods for delivery and/or deployment. Certain embodiments disclosed herein may be used in procedures to treat May-Thurner syndrome and/or deep venous thrombosis and the resulting post-thrombotic syndrome.
Description of the Related Art
May-Thurner syndrome, also known as iliac vein compression syndrome, is a condition in which compression of the common venous outflow tract of the left lower extremity may cause various adverse effects, including, but not limited to, discomfort, swelling, pain, and/or deep venous thrombosis (DVT) (commonly known as blood clots). May-Thurner syndrome occurs when the left common iliac vein is compressed by the overlying right common iliac artery, leading to stasis of blood, which may cause the formation of blood clots in some individuals. Other, less common, variations of May-Thurner syndrome have been described, such as compression of the right common iliac vein by the right common iliac artery.
While May-Thurner syndrome is thought to represent between two to five percent of lower-extremity venous disorders, it frequently goes unrecognized. Nevertheless, it is generally accepted that May-Thurner syndrome is about three times more common in women than it is in men and typically manifests itself between the age of twenty and forty. Patients exhibiting both hypercoagulability and left lower extremity thrombosis may be suffering from May-Thurner syndrome. To confirm that diagnosis, it may be necessary to rule out other causes for hypercoagulable state, for example by evaluating levels of antithrombin, protein C, protein S, factor V Leiden, and prothrombin G20210A.
By contrast to the right common iliac vein, which ascends almost vertically parallel to the inferior vena cava, the left common iliac vein takes a more transverse course. Along this course, it lies under the right common iliac artery, which may compress it against the lumbar spine. Iliac vein compression is a frequent anatomic variant—it is thought that as much as 50% luminal compression of the left iliac vein occurs in a quarter of healthy individuals. However, compression of the left common iliac vein becomes clinically significant only if such compression causes appreciable hemodynamic changes in venous flow or venous pressure, or if it leads to acute or chronic deep venous thrombosis, which will be discussed in more detail below. In addition to the other problems associated with compression, the vein may also develop intraluminal fibrous spurs from the effects of the chronic pulsatile compressive force from the overlying artery.
The narrowed, turbulent channel associated with May-Thurner syndrome may predispose the afflicted patient to thrombosis. And, the compromised blood flow often causes collateral blood vessels to form—most often horizontal transpelvis collaterals, connecting both internal iliac veins to create additional outflow possibilities through the right common iliac vein. Sometimes vertical collaterals are formed, most often paralumbar, which can cause neurological symptoms, like tingling and numbness.
Current best practices for the treatment and/or management of May-Thurner syndrome is proportional to the severity of the clinical presentation. Leg swelling and pain is best evaluated by vascular specialists, such as vascular surgeons, interventional cardiologists, and interventional radiologists, who both diagnose and treat arterial and venous diseases to ensure that the cause of the extremity pain is evaluated. Diagnosis of May-Thurner syndrome is generally confirmed one or more imaging modalities that may include magnetic resonance venography, and venogram, which, because the collapsed/flattened left common iliac may not be visible or noticed using conventional venography, are usually confirmed with intravascular ultrasound. To prevent prolonged swelling or pain as downstream consequences of the left common iliac hemostasis, blood flow out of the leg should be improved/increased. Early-stage or uncomplicated cases may be managed simply with compression stockings. Late-stage or severe May-Thurner syndrome may require thrombolysis if there is a recent onset of thrombosis, followed by angioplasty and stenting of the iliac vein after confirming the diagnosis with a venogram or an intravascular ultrasound. A stent may be used to support the area from further compression following angioplasty. However, currently available stenting options suffer from several complications—including severe foreshortenting, lack of flexibility (which can force the vessel to straighten excessively), vessel wear and eventual performation, increased load on and deformation of the stent causing early fatigue failure, and/or impedence of flow in the overlying left iliac artery potentially causign peripheral arterial disease. The compressed, narrowed outflow channel present in May-Thurner syndrome may cause stasis of the blood, which an important contributing factor to deep vein thrombosis.
Some patients suffering from May-Thurner syndrome may exhibit thrombosis while others may not. Nevertheless, those patients that do not experience thrombotic symptoms may still experience thrombosis at any time. If a patient has extensive thrombosis, pharmacologic and/or mechanical (i.e., pharmacomechanical) thrombectomy may be necessary. The hemostasis caused by May-Thurner syndrome has been positively linked to an increased incidence of deep vein thrombosis (“DVT”).
Deep vein thrombosis, or deep venous thrombosis, is the formation of a blood clot (thrombus) within a deep vein, predominantly in the legs. The right and left common iliac are common locations for deep vein thrombosis, but other locations of occurrence are common. Non-specific symptoms associated with the condition may include pain, swelling, redness, warmness, and engorged superficial veins. Pulmonary embolism, a potentially life-threatening complication of deep vein thrombosis, is caused by the detachment of a partial or complete thrombus that travels to the lungs. Post-thrombotic syndrome, another long-term complication associated with deep venous thrombosis, is a medical condition caused by a reduction in the return of venous blood to the heart and can include the symptoms of chronic leg pain, swelling, redness, and ulcers or sores.
Deep vein thrombosis formation typically begins inside the valves of the calf veins, where the blood is relatively oxygen deprived, which activates certain biochemical pathways. Several medical conditions increase the risk for deep vein thrombosis, including cancer, trauma, and antiphospholipid syndrome. Other risk factors include older age, surgery, immobilization (e.g., as experienced with bed rest, orthopedic casts, and sitting on long flights), combined oral contraceptives, pregnancy, the postnatal period, and genetic factors. Those genetic factors include deficiencies with antithrombin, protein C, and protein S, the mutation of Factor V Leiden, and the property of having a non-O blood type. The rate of new cases of deep vein thrombosis increases dramatically from childhood to old age; in adulthood, about 1 in 1000 adults develops the condition annually.
Common symptoms of deep vein thrombosis include pain or tenderness, swelling, warmth, redness or discoloration, and distention of surface veins, although about half of those with the condition have no symptoms. Signs and symptoms alone are not sufficiently sensitive or specific to make a diagnosis, but when considered in conjunction with known risk factors can help determine the likelihood of deep vein thrombosis. Deep vein thrombosis is frequently ruled out as a diagnosis after patient evaluation: the suspected symptoms are more often due to other, unrelated causes, such as cellulitis, Baker's cyst, musculoskeletal injury, or lymphedema. Other differential diagnoses include hematoma, tumors, venous or arterial aneurysms, and connective tissue disorders.
Anticoagulation, which prevents further coagulation but does not act directly on existing clots, is the standard treatment for deep vein thrombosis. Other, potentially adjunct, therapies/treatments may include compression stockings, selective movement and/or stretching, inferior vena cava filters, thrombolysis, and thrombectomy.